Sonography of the normal thymus by Adam and Ignotus in 50 children 2-8 years old with the Diasonics DRF scanner and a 7.5-MHz probe found that the thymus could be completely visuallized in 97% of subjects. The mean ateroposterior and longitudinal measurements were 1.4 and 2.5 cm respectively. The right lobe tended to have an antiverted teardrop shape while the left lobe was either triangular or sickle shaped. They found that the mean dimensions of the thymus changed little with age between 2 and 8 years, and the internal echogenicity resembles that of the liver. Mayilyn Siegel gives sonogrphic measurements of between 0.81 and 2.35cm for the right lobe in the anteriorposterior dimension and between 1.54 and 4.02cm in the longitudinal direction. The left lobe is 0.78 to 2.47 in the anteriorposterior direction, and 1.71 to 4.1 longitudinally.

Heifberg et al performed a retrospective study reviewing the chest CT scans of 40 subjects younger than 20 years of age, and found that the gland had a smooth, undulating, lateral contours tending to be convex laterally in the very young and concave laterally in the older subjects. The posterior border of the gland molded to the heart and great vessels, and the anterior contour molded to the chest wall and had a point shape directed toward the sternum. According to Heiberg et al, attenuation readings of normal thymic tissue varied between patients, and within the gland. Measurements were between 20-119 Hounsfield units on precontrast scans and 28-185H on postcontrast scans. Heifberg did not give average measurements probably because of the great variation in size due to aging.

Marilyn Siegel reports that the thymus should be visible in all normal individuals under age 20 on CT. In infants and children is extends from the level of the left brachiocephalic vein cephalad to the origin of the great vessels caudally. In young infants the thymus can extend to the diaphram. In children under 5 years of age the thymus has a quadrilateral shape with convex lateral margins. In children over 5, it assumes a triangular or arowhead shape with strait or concave margins. After puberty, areas of heterogeneity due to fatty infiltration begin to apear, and it begins to decrease in size. The mean thickness on CT of patients younger than 10 is 1.50cm, and for patients over 10 years of age is1.05. Thickness is the best indicator of infiltrative disease.

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